1. Field of the Invention
The present invention generally relates to a method for sensing an amount of moisture on a surface.
2. Description of the Related Art
Various methods for sensing an amount of moisture on a surface are known in the art. One such method is shown in U.S. Pat. No. 5,432,415 (the '415 patent). To sense moisture on the surface of a window, the method of the '415 patent includes a step of transmitting a transmitter signal having a first phase to generate a wave. The method continues with a step of propagating the wave along the window to a receiver to vibrate the window. The method next includes a step of converting the wave into a receiver signal having a second phase. Since the amount of moisture on the window corresponds to variation in the second phase relative to the first phase, the method of the '415 patent includes a step of comparing the first phase to the second phase. Once compared, the method includes a step of comparing the difference between the first phase and the second phase to a threshold value. The threshold value represents a minimum amount of moisture on the windshield necessary to enable the wiper system. The method further includes a step of actuating a wiper system to remove the moisture from the surface if the difference between the first phase and the second phase exceeds the threshold value.
Although the sensing system of the '415 patent can provide an approximation of moisture on the surface, it lacks the ability to finely sense the amount of moisture on the surface because it does not incorporate the ability to compensate for factors that affect phase shift other than moisture by accounting for factors other than moisture. An example of such a factor is the temperature of the window. If not properly incorporated in the moisture estimation system calculation, the temperature of the window may cause either a “false positive” for moisture on the surface and needlessly operate the wipers or a “false negative” and not operate the wipers when there is moisture on the surface. Hence, there remains an opportunity for a method of determining the amount of moisture on the surface that compensates for the temperature of the surface in determining the amount of moisture on the surface and thus yields a more robust system.